The invention relates to an electrically controlled fuel injection pump for internal combustion engines as generically defined hereinafter. In such a pump, known from U.S. Pat. No. 4,392,612, which as a unit fuel injector is built directly into the cylinder head of the associated engine and in which both the mechanically driven fuel injection pump and the associated injection nozzle are accommodated in a common housing, the fuel injection quantity that is positively displaced from the pump work chamber to the injection nozzle during the supply stroke of the pump piston is determined by the ON time of an electromagnetically actuated overflow valve that is open when it is free of electric current. This overflow valve is inserted into an overflow conduit joining the pump work chamber and a low-pressure chamber and is embodied as a slide valve that opens outward. In the closed state, it is balanced in pressure, because the fuel that has been put under injection pressure in the pump work chamber acts upon a pressure chamber formed by an annular groove on the valve needle shank and defined toward the outside by the guide bore, so that no axial forces can be exerted. If this valve is switched over into its open position, however, with the adjusting magnet not in the excited state and with the aid of a compression spring biased in the opening direction, then the expanding fuel in the low-pressure chamber shoots through the narrow gap between the conical valve seat and the corresponding conical closing surface, the latter widening toward the outside, at the remote end section of the valve needle in terms of the adjusting member. In this process, additional hydraulic forces are exerted; they reinforce the opening movement but, disadvantageously, because of the restriction in the flow, they also generate counter forces and lead to what is known as valve buzzing. This effect is due to a slight fluttering movement of the valve member, and while it is desirable in injection valves, it is disadvantageous in control valves.
Fuel injection pumps of a similar type are also known, which instead of an outwardly opening overflow valve have an inwardly opening magnetic valve to control the onset and end of fuel supply (U.S. Pat. Nos. 1,664,608 and 4,129,253). In these valves, the electromagnet which acts as the electrical control member must firmly hold the valve member in the closing position, counter to the injection pressure which is exerted centrally on the valve needle tip. This valve cannot be realized such that it is pressure-balanced, and at high injection pressures the adjusting magnet is capable of keeping the valve in its closing position only if the seat diameter is selected to be very small. This is attained, in the known fuel injection pumps, by means of a preceding throttle bore disposed between the pump work chamber and the valve seat. This provision has the disadvantage, however, that the outflow speed, as well, of the fuel flowing out of the pump work chamber at the end of injection is throttled, which is a hindrance to achieving the sharply-defined, rapid end of injection that is desired.
From German Offenlegungsschrift No. 31 39 669, a fast-switching magnetic valve is also known, which can also be used for controlling injection and which generates a very large closing force by means of two axial air gaps axially offset from one another. This valve, too, has the disadvantage of the valve embodiment used in the other pumps discussed above, namely that in the closed state, because of the injection pressure being exerted upon the valve seat surface, it is not pressure-balanced and can thus be used only up to a limited pressure level, for instance for controlling a gasoline injection pump.